Circuit breaker panelboard with grounded neutral protection

ABSTRACT

Circuit breaker panelboard assembly in which each breaker includes ground fault responsive means, and means is provided externally of the breakers for preventing the ground fault responsive means for being rendered ineffective by grounding of the neutral conductor in one of the branch circuits served by the panelboard assembly.

United States Patent 1 Polley et al.

[ 11 3,745,415 1 July 10, 1973 CIRCUIT BREAKER PANELBOARD WITH GROUNDEDNEUTRAL PROTECTION Inventors: Bruce T. Polley, Vallejo; Wallace W.Wahlgren, Oakland, both of Calif.

Assignee: The Rucker Company, Oakland,

Calif.

Filed: Oct. 24, 1972 Appl. No.2 299,816

US. Cl. 317/18 D, 317/26 Int. Cl. H02h 3/28 Field of Search 317/18 D, 26

References Cited UNITED STATES PATENTS 11/1970 Stevenson 317/18 D3,611,038 10/1971 Benhan ..317/18D Primary Examiner-James D. TrammellAttorney-Paul D. Flehr, Harold C. Hohach et al.

[5 7 ABSTRACT Circuit breaker panelboard assembly in which each breakerincludes ground fault responsive means, and means is provided externallyof the breakers for preventing the ground fault responsive means forbeing rendered ineffective by grounding of the neutral conductor in oneof the branch circuits served by the panelboard assembly.

10 Claims, 4 Drawing Figures CIRCUIT BREAKER PANELBOARD WITH GROUNDEDNEUTRAL PROTECTION BACKGROUND OF THE INVENTION This invention pertainsgenerally to electrical safety devices and more particularly to acircuit breaker panelboard assembly having built-in ground faultprotection.

Circuit breaker panelboard assemblies are commonly used at theelectrical service entrance of buildings for distributing current frommain lines feeding the building to branch circuits within the building.A separate circuit breaker is generally provided for each branchcircuit, and the breaker usually includes means for disconnecting thecircuit from the main lines in the event of an overcurrent condition(i.e., a current on the order of I to 300 percent of the nominal ratingof the breaker) or a short circuit condition (i.e., a current in excessof approximately 300 percent of the nominal rating). The neutralconductor of the main lines is normally connected to ground at itssource end, and recently some circuit breakers intended for use inpanelboard assemblies have been provided with ground fault responsivemeans for protecting human life and prop erty against electrical shockand damage resulting from contact with or grounding of one of the lineconductors in a branch circuit. Such breakers typically include adifferential transformer having a magnetic core and primary windings ofat least one turn connected in series with the line and neutralconductors. The trans former also includes a secondary winding which isconnected to means for giving a visual indication of a ground faultand/or interrupting the flow current in the distribution system inresponse thereto. Under normal conditions, the total current flowingthrough the line conductor is equal to the current flowing through theneutral conductor, producing a zero net magnetic flux in thedifferential transformer core and no output from the secondary winding.Upon the occurrence of a ground fault, the current in the line conductordiffers from that in the neutral conductor by an amount corresponding tothe magnitude of the fault current. This difference produces a nonzeronet magnetic flux in the transformer core and induces a fault signal inthe secondary winding. Y

While devices of the above character operate satisfactorily under mostconditions, their operation is seriously impaired by the occurrence of alow impedance grounding of the neutral conductor between thedifferential transformer and the load. Under normal fault conditions,the current produced by a ground fault returns to the source via groundconductors such as grounding wires, metal conduits or pipes, metalbuilding frames, wet insulation, water, damp earth, faultyinstallations, damaged appliances or equipment and the like, bypassingthe portion of the neutral conductor which passes through thedifferential transformer. Grounding of the neutral conductor on the loadside of the differential transformer provides an alternate path to thesource for a ground fault current, causing only a portion of the currentto return to the source through the neutral conductor and thedifferential transformer. This substantially reduces the sensitivity ofthe transformer and makes it impossible to insure the level of faultcurrent required to produce tripping, the increase in fault currentrequired being dependent upon the magnitude of the impedance between theneutral conductor and ground. In addition, grounding of the neutralconductor on both sides of the transformer causes the neutral to acteffectively as a shorted secondary winding which reduces theeffectiveness of the transformer and makes it difficult to insure thelevel of fault current which will produce tripping.

Heretofore, in order to prevent the ground fault responsive means incircuit breakers from being rendered inoperative by grounding of theneutral conductor in the branch circuit, the breakers have each beenprovided with means for detecting grounding of the neutral conductor.Such means has been incorporated in and forms an integral part of eachbreaker in the panelboard.

SUMMARY AND OBJECTS OF THE INVENTION In the panelboard assembly of thepresent invention, each circuit breaker includes ground faultprotection, and grounded neutral protection is provided externally ofthe breakers. A transformer mounted in the panelboard has a normallyenergized primary winding and a plurality of secondary windingsconnected electrically in series with conductors leading to the neutralconductors in the branch circuits. In the event that one of theseconductors becomes grounded, the transformer induces an additionalcurrent in it, and this current causes the breaker for the particularcircuit to trip.

It is in general an object of the invention to provide a new andimproved circuit breaker panelboard assembly having ground faultprotection and means for preventing the ground fault protection frombeing ren dered inoperative by grounding of the neutral conductor.

Another object of the invention is to provide a panelboard assembly ofthe above character in which the grounded neutral protection is providedexternally of the circuit breakers.

Another object of the invention is to provide a more reliable signalthat an extraneous ground exists in a circuit.

Another object of the invention is to eliminate some of the componentsand circuitry from the ground fault protected breakers, making them lessexpensive to build.

Another object of the invention is to reduce the overall cost of circuitbreaker panelboards incorporating ground fault protection.

Additional objects and features of the invention will be apparent fromthe following description in which the preferred embodiments are setforth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic illustration ofone embodiment of a panelboard assembly with ground fault and groundedneutral protection incorporating the present invention.

FIGS. 2-4 are schematic illustrations of other forms of transformerconstructions which can be utilized in the panelboard assembly of FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings, the inventionis illustrated in connection with a single-phase, three-wire main linesystem consisting of line conductors L1 and L2 and a neutral conductor Nwhich is connected to ground at its source end. The panelboard assemblyis enclosed in an enclosure which in the preferred embodiment isfabricated of sheet metal and grounded. Line buses 11 and 12 and aneutral bus 13 are provided within the enclosure and adaptedrespectively for connection to line conductors L1 and L2 and neutralconductor N.

A plurality of circuit breakers 16-19 are mounted in the enclosure andcontrol the flow of current from the line and neutral buses to aplurality of branch circuits 21-24. Each of these circuits includes lineand and neutral conductors designated respectively by the letters L andN. Breakers 16 and 17 have input terminals which releasably engage linebus 11 and pigtails 26 and 27 which are connected to neutral bus 13.Breakers l8 and 19 likewise have input terminals which engage line bus12 and pigtails 28 and 29 which connect to neutral bus 13. Thus,breakers 16 and 17 control the flow of current between main lineconductors L1 and N and branch circuit conductors 21L, 21N, 22L and 22N,while breakers l8 and 19 control theflow between main line conductors L2and N and branch circuit conductors 23L, 23N, 24L, and 24N. Forconvenience of illustration, only four breakers are shown in thedrawing. However, it will be understood that additional breakers can beconnected in parallel with those shown for controlling the flow ofcurrent to additional branch circuits.

Each of the circuit breakers in the assembly has builtin ground faultprotection in addition to short circuit and over current protection.Breakers of this type are described in detail in copending applicationSer. NO. 141,298, filed May 7, 1971 and assigned to the assignee herein.Briefly, these breakers include a differential transformer formonitoring the flow of current in the line and neutral conductors, a setof contacts for controlling the flow of current in the conductors, andmeans for opening the contacts in response to a short circuit, anovercurrent or an imbalance in the currents monitored by thedifferential transformer.

Means is provided for preventing the ground fault responsive means inthe circuit breakers from being defeated by grounding of the neutralconductors in the branch circuit. This'means includes a transformer 31having a core 32 of magnetic material defining a window 32a. A primarywinding 33 is wound on the core and energized from the main lines. Oneend of the winding is connected directly to neutral bus 13, and theother end is connected to line bus 11 through a resistor 34. Thisresistor serves as a current limiting resistor, and it is preferablychosen to have a value corresponding to the resistance of the primarywinding so that approximately one half of the line voltage is applied tothe winding. Pigtails 26-29 pass through core window 32a to form singleturn secondary windings. Since the currents in line conductors L1 andL2are out of phase with each other, the neutral pigtails associated withthese conductors pass through the core window in opposite directions sothat the ampere turns produced by the pigtails will be additive ratherthanjof subtractive polarity-Thus, pigtails 26 and 27 pass through thecore in one direction, and pigtails 28 and 29 pass through the core inthe opposite direction. In the preferred embodiment, the number of turnsin primary winding 33, the size of resistor 34, and the cross-sectionalarea of core 32 are chosen to be such that the core operates nearsaturation when excited by primary winding 33 and induces a secondaryvoltage on the neutral of approximately 50 millivolts. The direction ofpigtails 26-29 is such that any current flowing therein causes the coreto saturate and hence increases the secondary theory induced voltage asmall percentage. Resistor 34 limits the primary current to a safevalue, and the surface area of the core is made sufficiently large todissipate the heat of saturation.

FIG. 2 illustrates another form of transformer construction 36 which canbe used in the panelboard assembly of FIG. 1. This transformer has acore 37 which is formed to include a pair of windows 37a and 37b.Pigtails 26 and 27 pass through windows 37a, and the pigtails 28 and 29pass through windows 3712, with all of the pigtails passing through thecore in the same direction. Primary winding 33 is wound on the centralleg of core 37. This construction provides the same phase relationshipsas the single window structure shown in FIG. 1.

FIGS. 3 and 4 illustrate two additional transformer constructions whichcan be utilized in the panelboard assembly of FIG. 1. These transformersare generally similar to those shown in FIGS. 1 and 2, respectively,except that primary-winding 33 is formed in two parts connectedelectrically in series, as for example, by a center tab 38. The centertab is connected to neutral bus 13, and one end of the winding isconnected to line bus 11 through resistor 34. .The other end of thewinding is connected to line bus 12 through a resistor 39. Pigtails26-29 pass through the core windows as before.

If desired, in any of the transformer constructions illustrated, primarywinding 33 can be energized from a source other than the main lineconductors. For example, this winding can be energized by a highfrequency pulser of the type shown and described in US. Pat. No.3,611,035 issued Oct. 5, 1971 to the assignee herein.

Operation and use of the panelboard assembly can be described briefly.In the absence of a ground fault, short circuit or overcurrent, thecontacts of circuit breakers 16-19 are closed, and current passes fromthe main lines to the conductors of the branch circuits. In the event ofa ground fault in one of the branch circuits, as would occur, forexample, if a person accidentally contacted conductor 21L, the circuitbreaker for that circuit would detect an imbalance in the current in theline and neutral conductors and disconnect the circuit from the mainlines.

During normal conditions, primary winding 33 is energized, but noadditional current is produced in any of the neutral conductors as longas these conductors remain ungrounded on the load side. In the eventthat one of the branch circuit neutral conductors becomes grounded, aninduced current will flow in that conductor because of the secondaryvoltage of the transformer, producing an imbalance which causes thebreaker for the circuit to trip. The other breakers remain closed, andtheir circuits continue to receive power.

The invention has a number of important features and advantages. Itprevents the normal operation of a circuit breaker with ground faultprotection from being defeated by inadvertent grounding of the neutralconductor. It provides individual protection for each branch circuit,yet avoids unnecessary duplication of parts. Moreover, it can readily beincorporated in existing panelboards.

It is apparent from the foregoing that a new and improved panelboardassembly has been provided. While only the presently preferredembodiments have been described, as will be apparent to those familiarwith the art, certain changes and modifications can be made withoutdeparting from the scope of the invention as defined by the followingclaims. I

We claim:

1. In a circuit breaker panel assembly for distributing current frommain lines to a plurality of branch circuits, said main lines and eachof said branch circuits including at least one line conductor and aneutral conductor, the neutral conductor of said main lines beingconnected to ground at its source end, an enclosure, line and neutralterminals in the enclosure for connection to the line and neutralconductors of the main line, a plurality of circuit breakers connectedto the line and neutral terminals and adapted for connection to the lineand neutral conductors of the branch circuit for controlling the flow ofcurrent from the main lines to the branch circuits, each of said circuitbreakers including ground fault responsive means for monitoring the flowof current through the same and interrupting said flow in the event ofan imbalance in the current, and means mounted externally of the circuitbreakers for inducing an additional current in the neutral conductors inthe event that one of the neutral conductors in a branch circuit isgrounded.

2. An assembly as in claim 1 wherein the circuit breakers are connectedto the neutral terminals by means of conductors passing through a coreof magnetic material, 'said core having a winding thereon connected to asource of electric energy.

3. An assembly as in claim 2 wherein the winding is connected to theline and neutral terminals.

4. An assembly as in claim 3 together with a current limitingresistorconnected electrically in series with the winding.

5. A circuit breaker panel assembly for distributing current from mainlines to a plurality of branch circuits, Said main lines and each ofsaid branch circuits including at least one line conductor and a neutralconductor, the neutral conductor of said main lines being connected toground at its source end, an enclosure, line and neutral terminals inthe enclosure for connection to the line and neutral conductors of themain lines, a plurality of circuit breakers connected to the line andneutral terminals and adapted for connection to the line and neutralconductors of the branch circuit for controlling the flow of currentfrom the main lines to the branch circuits, each of said circuitbreakers including ground fault responsive means for monitoring the flowof current through the same and interrupting said flow in the event ofan imbalance in the current, and a transformer having a primary windingconnected to a source of electrical energy and a plurality of secondarywindings connected electrically in series with the neutral conductors.

6. An assembly as in claim 5 wherein the transformer includes a core ofmagnetic material and the circuit breakers are connected to the neutralterminal by means of conductors passing through said core.

7. An assembly as in claim 6 wherein first and second line terminals areprovided for connection to first and second line conductors in the mainlines, a first group of the circuit breakers is connected to the firstline terminal, a second group of the circuit breakers is connected tothe second line terminal, and the conductors connecting the first andsecond groups of the circuit breakers to the neutral terminal passthrough the transformer core in opposite directions.

8. An assembly as in claim 6 wherein first and second line terminals areprovided for connection to first and second line conductors in the mainlines, a first group of the circuit breakers is connected to the firstline terminal, a second group of the circuit breakers is connected tothe second line terminal, the transformer core has two separate windowsand the conductors connecting the first and second groups of circuitbreakers to the neutral terminal pass through different ones of saidwindows.

9. An assembly as in claim 5 wherein the primary winding is connected tothe line and neutral terminals.

10. An assembly as in claim 9 together with a current limiting resistorconnected electrically in series with the primary winding.

1. In a circuit breaker panel assembly for distributing current frommain lines to a plurality of branch circuits, said main lines and eachof said branch circuits including at least one line conductor and aneutral conductor, the neutral conductor of said main lines beingconnected to ground at its source end, an enclosure, line and neutralterminals in the enclosure for connection to the line and neutralconductors of the main line, a plurality of circuit breakers connectedto the line and neutral terminals and adapted for connection to the lineand neutral conductors of the branch circuit for controlling the flow ofcurrent from the main lines to the branch circuits, each of said circuitbreakers including ground fault responsive means for monitoring the flowof current through the same and interrupting said flow in the event ofan imbalance in the current, and means mounted externally of the circuitbreakers for inducing an additional current in the neutral conductors inthe event that one of the neutral conductors in a branch circuit isgrounded.
 2. An assembly as in claim 1 wherein the circuit breakers areconnected to the neutral terminals by means of conductors passingthrough a core of magnetic material, said core having a winding thereonconnected to a source of electric energy.
 3. An assembly as in claim 2wherein the winding is connected to the line and neutral terminals. 4.An assembly As in claim 3 together with a current limiting resistorconnected electrically in series with the winding.
 5. A circuit breakerpanel assembly for distributing current from main lines to a pluralityof branch circuits, said main lines and each of said branch circuitsincluding at least one line conductor and a neutral conductor, theneutral conductor of said main lines being connected to ground at itssource end, an enclosure, line and neutral terminals in the enclosurefor connection to the line and neutral conductors of the main lines, aplurality of circuit breakers connected to the line and neutralterminals and adapted for connection to the line and neutral conductorsof the branch circuit for controlling the flow of current from the mainlines to the branch circuits, each of said circuit breakers includingground fault responsive means for monitoring the flow of current throughthe same and interrupting said flow in the event of an imbalance in thecurrent, and a transformer having a primary winding connected to asource of electrical energy and a plurality of secondary windingsconnected electrically in series with the neutral conductors.
 6. Anassembly as in claim 5 wherein the transformer includes a core ofmagnetic material and the circuit breakers are connected to the neutralterminal by means of conductors passing through said core.
 7. Anassembly as in claim 6 wherein first and second line terminals areprovided for connection to first and second line conductors in the mainlines, a first group of the circuit breakers is connected to the firstline terminal, a second group of the circuit breakers is connected tothe second line terminal, and the conductors connecting the first andsecond groups of the circuit breakers to the neutral terminal passthrough the transformer core in opposite directions.
 8. An assembly asin claim 6 wherein first and second line terminals are provided forconnection to first and second line conductors in the main lines, afirst group of the circuit breakers is connected to the first lineterminal, a second group of the circuit breakers is connected to thesecond line terminal, the transformer core has two separate windows andthe conductors connecting the first and second groups of circuitbreakers to the neutral terminal pass through different ones of saidwindows.
 9. An assembly as in claim 5 wherein the primary winding isconnected to the line and neutral terminals.
 10. An assembly as in claim9 together with a current limiting resistor connected electrically inseries with the primary winding.